I always felt that(1)it all depends on what you do to the gun and (2)what kind of gun it is- as to whether or not one has raised/lowered the value of a gun. Much like an antique car. If you took an old, original Fox in about 90% condition and put in screw in chokes, then I think the value of the gun will depreciated. But if you found the same gun in 25% condition and sent it to Doug Turnbull (or someone similar) to recondition the gun, you will definitely raise the value. A Model 12 with a 2" Moneymaker trap rib (I see one on ocassion) will go for a bit less than than the same gun in 95% condition. A BSS with screw in chokes added, won't really make a difference in my opinion. Just my opinion.